HELP please -additional shipping charges

[miov-45]

I recently received a notice from ebay that USPS claimed that some of my shipping costs were incorrect. Additonal fees were collected via my  credit card. Some of these charges were incorrect. What is my recourse? How can I prove that they are wrong when I no longer have the packages. FYI- when I measure and weigh I always round UP. In all the years that I have been selling- this has never happened

[disneyshopper]

When USPS claims you underpaid shipping, you receive a message from them.  There is a link in the message to dispute the extra charge.